The present invention is directed to a method and composition for use in inhibiting the formation and deposition of coke on surfaces during the elevated temperature processing of hydrocarbons. Coke deposition is generally experienced when hydrocarbon liquids and vapors contact the hot metal surfaces of the processing equipment. While perhaps not entirely technically understood, because of the complex makeup of the hydrocarbons, the hydrocarbons at elevated temperatures and in contact with hot metallic surfaces undergo various changes through either chemical reactions and/or decomposition of various unstable components of the hydrocarbon. The undesired products in many instances include coke, polymerized products, deposited impurities and the like. Whatever the undesired product that may be formed, the result is the smae, i.e., reduced economies of the process. If these deposits are allowed to remain unchecked, heat transfer, throughout and overall productivity are detrimentally effected. Moreover, downtime is likely to be encountered due to the necessity of either replacing and/or cleaning of the affected parts of the processing system.
While the formation and type of undesired products are dependent upon the hydrocarbon being processed and the conditions of the processing, it may generally be stated that such products can be produced at temperatures as low as 100.degree. F.; but are much more prone to formation as the temperature of the processing system and the metal surfaces thereof in contact with the hydrocarbon increase. At these temperatures, coke formation is likely to be produced regardless of the type hydrocarbon being charged. The type coke formed, i.e., amorphous, filamentous or pyrolytic, may vary somewhat; however, the probability of the formation of such is quite high.
As indicated in U.S. Pat. Nos. 3,531,394 and 4,105,540 the teachings of which are incorporated herein by reference, coke formation and deposition are common problems in ethylene (olefin) plants which operate at temperatures where the metal surfaces in contact with the hydrocarbon are at 1600.degree. F. and above. The problem is prevalent in the cracking furnace coils as well as in the transfer line exchangers where pyrolytic type coke formation and deposition is commonly encountered. Ethylene plants often referred to generally as "olefin plants", originally produced simple olefins such as ethylene, propylene, butanes and butadiene from a feed of ethane, propane, butanes and mixtures thereof. Later developments in the area of technology however, has led to the cracking of heavier feedstocks because of their availability to produce aromatics and pyrolsis gasoline as well as light olefins. Feedstocks now include light naphtha, heavy naphtha and gas oil.
According to the thermal cracking processes utilized in olefin plants, the feedstocks are cracked generally in the presence of steam in tubular pyrolysis furnaces. The feedstock is preheated, diluted with steam and the mixture heated in the pyrolysis furnace to about 1500.degree. F. and above, most often in the range of 1500.degree. to 1650.degree. F. The effluent from the furnace is rapidly quenched by direct means or in exchangers which are used to generate high pressure steam at 400 to 800 psig for process use. This rapid quench reduces the loss of olefins by minimizing secondary ractions. The cooled gas then passes to the prefractionator where it is cooled by circulating oil streams to remove the fuel oil fraction. In some designs, the gas leaving the quench exchanger is further cooled with oil before entering the prefractionator. In either case, the heat picked up by the circulating oil streams is used to generate steam and to heat other process streams. The mixture of gas and steam leaving the prefractionator is further cooled in order to condense the steam and most of the gasoline product in order to provide reflux for the prefractionator. Either a direct water quench or heat exchangers are used for this cooling duty.
After cooling, cracked gas at, or close to atmospheric pressure, is compressed in a multistage compression system to much higher pressures. There are usually four or five stages of compression with interstage cooling and condensate separation between stages. Most plants have hydrocarbon condensate stripping facilities. Condensate from the interstage knockout drums is fed to a stripper where the C.sub.2 hydrocarbons and lighter, are separated. The heavier hydrocarbons are fed to the depropanizer.
While various treatments have been proposed to eliminate or reduce filamentous coke formation at elevated temperatures, none have attained any great degree of success. In the book "Coke Formation on Metal Surfaces" by Albright and Baker, 1982, methods are described which utilize silicon and aluminum as pretreatments. In accordance with the procedure, the furnace tubes are pretreated with silicon and aluminum hours before introduction of the hydrocarbon feedstocks. With the use of silicon, furnace tubes are coated by the chemical vaporization of an alkoxysilane. While U.S. Pat. Nos. 4,105,540 and 4,116,812 are generally directed to fouling problems in general, the patents disclose the use of certain phsophate and phosphate and sulfur containing additives for use purportedly to reduce coke formation in addition to general foulants at high temperatue processing conditions.
With respect to coke retardation, various efforts have been reported, namely:
1. French Pat. No. 2,202,930 (Chem. Abstracts Vol. 83, 30687K) is directed to tubular furnace cracking of hydrocarbons where molten oxides or salts of group III, IV or VIII metals (e.g., molten lead containing a mixture of K.sub.3 VO.sub.4, SiO.sub.2 and NiO) are added to a pretested charge of, for example, naphtha/steam at 932.degree. F. This treatment is stated as having reduced deposit and coke formation in the cracking section of the furnace. PA1 2. Starshov et al, Izv Vyssh. Ucheban. Zaved., Neft GAZ, 1977 (Chem. Abst. Vol. 87: 154474r) describes the pyrolysis of hydrocarbons in the presence of aqueous solutions of boric acid. Carbon deposits were minimized by this process. PA1 3. Nikonov et al., U.S.S.R. No. 834,107, 1981; (Chem. Abst. 95:135651v) describes the pyrolytic production of olefins with peroxides present in a reactor, the internal surfaces of which have been pretreated with an aqueous alcoholic solution of boric acid. Coke formation is not mentioned in this patent since the function of the boric acid is to coat the inner surface of the reactor and thus decrease the scavenging of peroxide radicals by the reactor surface. PA1 4. Starshov et al., Neftekhimiya 1979 (Chem. Abst: 92:8645j) describes the effect of certain elements including boron on coke formation during the pyrolysis of hydrocarbons to produce olefins. PA1 5. U.S. Pat. No. 2,063,596 discusses in its prior art section the problems associated with the processing of hydrocarbons in equipment whose metallic parts have been supplied with a metalloid. The general impression is that such has not been utilized successfully. PA1 6. U.S. Pat. No. 1,847,095 in a somewhat ambiguous manner describes the use of metalloid compounds such as boron hydride which are capable of yielding "volatile hydrogen" during the processing of hydrocarbons. The patent is silient as regards the type of coke encountered and the problems associated therewith and contains no disclosure or suggestion relative to other boron compounds which may be utilized during the processing of hydrocarbons for protection against coke formation. PA1 7. Baker, R. T. K., Gas Chem. Nucl. React. Large In dust. Plant, Proc. Conf., 1980. Chem. Ab. Vol. 94, 1981, 94:8141h, is directed to the role of various additives e.g., B.sub.2 O.sub.3 in effecting the growth rate of filamentous coke produced from the decomposition of C.sub.2 H.sub.2 on Ni-Fe or Mo Catalysts. B.sub.2 O.sub.3 is stated as being the only additive which failed to provide any significant reduction in the growth of the filaments.